doc: Update 'Kubespray vs Kubeadm' (kubernetes-sigs#7834)

non-kubeadm mode has been removed since ddffdb6
2.5 years ago. The non-kubeadm makes unnecessary confusion today, then
this updates the documentation.

docs/comparisons.md

@@ -21,7 +21,6 @@ does generic configuration management tasks from the "OS operators" ansible
 world, plus some initial K8s clustering (with networking plugins included) and
 control plane bootstrapping.
 
-Kubespray supports `kubeadm` for cluster creation since v2.3
-(and deprecated non-kubeadm deployment starting from v2.8)
+Kubespray has started using `kubeadm` internally for cluster creation since v2.3
 in order to consume life cycle management domain knowledge from it
 and offload generic OS configuration things from it, which hopefully benefits both sides.